1. Field of the Invention
The present invention relates to a liquid crystal display, and, more particularly, to a semi-transmission type liquid crystal display which reflects incident light coming from outside to provide a display light source and transmits light from a light source at the back.
2. Description of the Related Art
There is a reflection type liquid crystal display (LCD) known which has a reflector inside and reflects incident external light by the reflector to provide a display light source, thereby eliminating the need for a backlight as a light source and a transmission type liquid crystal display equipped with a backlight as a light source.
As the reflection type liquid crystal display can be designed with lower power consumption, thinner and lighter than the transmission type liquid crystal display, it is mainly used for a portable terminal. This is because as light input from outside is reflected at the reflector in the display, the light can be used as a display light source, thus eliminating the need for a backlight. The transmission type liquid crystal display has such a characteristic as having a better visibility than the reflection type liquid crystal display in case where ambient light is dark.
As a reflector which improves the luminance of a reflection type liquid crystal display, there is one which, for example, has isolated projections formed on the surface of the reflector leaving an organic insulating film in a photolithography process, and an interlayer film is provided on the projections, thereby yielding a smooth undulated shape with mountain portions comprised of the projections and the other or valley portions, so that an undulation pattern is formed on the surface of the reflector (see Japanese Patent No. 2825713).
FIG. 29 is a plan view showing an example of the undulation pattern formed on the conventional reflector. shown in FIG. 29, the undulation pattern is formed by arranging a plurality of projections 2 with a circular planar shape as a projection pattern or a base pattern on the surface of a reflector 1 in an isolated state. The undulation pattern causes irregular reflection of incident light, thus improving the luminance of the liquid crystal display.
The basic structure of the existing liquid crystal displays comprises a liquid crystal of an TN (Twisted Nematic) type, a single sheet polarizer type, an STN (Super Twisted Nematic) type, a GH (Guest-Host) type, a PDLC (Polymer Dispersed Liquid Crystal) type, a cholesteric type or the like, a switching element which drives the liquid crystal and a reflector or backlight provided inside or outside a liquid crystal cell. Those ordinary liquid crystal displays employ an active matrix drive system which can achieve high definition and high image quality using thin film transistors (TFTS) or metal/insulating film/metal structure diodes (MIMs) as switching elements, and are equipped with a reflector or backlight.
As a liquid crystal display which has advantages of both the conventional reflection type liquid crystal display and transmission type liquid crystal display, a semi-transmission type liquid crystal display is disclosed (see Japanese Patent No. 2955277) which, as shown in FIG. 30, has gate interconnections 4 and source interconnections 5 so provided as to run around pixel electrodes 3 of an active matrix substrate and intersect each other perpendicularly, has thin film transistors 6 provided on the pixel electrodes 3, has the gate interconnections 4 and source interconnections 5 connected to the gate electrodes and source electrodes of the thin film transistors 6 and has reflection areas 7 of a metal film and transmission areas 8 of ITO formed in the pixel electrodes 3.
As the reflection areas and transmission areas are provided in the pixel electrodes, the backlight can be turned off when the ambient light is bright so that the liquid crystal display can be used as a reflection type liquid crystal display, and thus demonstrates lower power consumption that is the characteristic of the reflection type liquid crystal display. When the ambient light is dark, the backlight is turned on so that the liquid crystal display is used as a transmission type liquid crystal display, and thus demonstrates an improved visibility in case where ambient light is dark, which is the characteristic of the transmission type liquid crystal display. Hereunder, a liquid crystal display which can be used as a reflection type liquid crystal display and as a transmission type liquid crystal display will be called as a semi-transmission type liquid crystal display.
In the liquid crystal display described in Japanese Patent No. 2955277, however, as shown in FIG. 31, undulations formed on an active matrix substrate are partly removed for planarization and transmission areas on the pixel electrodes are formed on the flat portion of the active matrix substrate. The undulations formed on the active matrix substrate are provided to efficiently reflect ambient light toward a user. In case where the areas of the undulations are reduced to form the transmission areas so that the liquid crystal display is used as a reflection type liquid crystal display with the backlight turned off, there arises a problem that the luminance drops.
Japanese Patent Laid-Open No. 2001-75091 describes a reflector which has isolated projections formed on the surface of the reflector by combining the above-described two prior arts, thereby forming an undulation pattern on the surface of the reflector, has openings formed in the top portions and bottom portions of the undulation pattern and uses openings as transmission areas. Because the projections have isolated circular shapes, however, the reflector simultaneously reflects incident lights from all the directions and has no directivity, the display luminance is undesirably reduced.
Because separation of the transmission areas from the reflection areas in the liquid crystal display described in Japanese Patent No. 2955277 is simple, it is easy to form color filters with different thicknesses on the opposite substrate for different areas. As the reflector described in Japanese Patent Laid-Open No. 2001-75091 has transmission areas and reflection areas mixed in pixels, however, it is difficult to form color filters with different thicknesses on the opposite substrate for different areas. This makes it impossible to adjust the thicknesses of the color filters area by area. Accordingly, light passes the color filter formed on the opposite substrate twice in reflection mode but passes it once in transmission mode. This changes the hue in transmission mode and reflection mode, so that the reduction in luminance and the change in hue lower the visibility.